Intercommunication system including a master station



K. w. GRAYBILL Filed June 1:5, 1945 INTERCOMMUNICATION SYSTEM INCLUDING A MASTER STATION "Sept. 3

JNVENTOR. Kenneth W. Graybi I I ATTORNEY station.

ployed to permit selective signaling of the sub- Patented Sept. 30, 1947 INTERCOMMUNICATION SYSTEM INCLUD- ING A MASTER- STATION Kenneth Wayne Graybill, Elmhurst, 111., assignor to Automatic Electric Laboratories, Inc., -hicago, 111., a corporation of Delaware Application June 13, 1945, Serial No. 599,169

" The present invention pertains in general to telephone systems of the intercommunicating ty e and, in particular, to a system including a master station and a plurality of subordinate stations.

The principal object of the invention is to provide a telephone system of the above described character which requires a minimum number of interstation conductors and which yet provides separate, oppositely directed, speech channels between the master station and a subordinate station so as to permit the use of loudspeaking equipment at the master station without amplification.

A further object of the invention is to provide a telephone system in accordance with the preceding object wherein means are provided at the master station for disabling the speech channel outgoing therefrom in order to prevent eavesdropping by a subordinate station.

A feature of the invention is a combination push and turn key which is arranged to permit operation of either of two sets of switching contacts, one set being used to signal a subordinate station and the other set being used to disable the transmitter at the master station.

In accordance with the invention, a common talking circuit is employed to convey speech signals from the master station to any of the subordinate stations and talking circuits individual to the subordinate stations are employed to convey speech signals therefrom to the master The individual circuits are also emordinate stations from the master station.

In the accompanying drawing illustrating the invention, Fig. 1 is a schematic diagram showing a master station and three subordinate stations and Fig. 2 illustrates the mechanical construction of the station selecting push keys, one of which is adapted to operate an auxiliary set of contacts when rotated.

Referring now to Fig. 1, the master station is designated A and the three subordinate stations are designated B, C, and D. The master station A comprises a sensitive transmitter or microphone I, a buzzer 2, a loudspeaking receiver 3 and an associated impedance matching transformer 4, a condenser 5, a plurality of non-locking station selecting push keys b, c, and d, and

9 Claims. (01. 1791) a locking turn key'b which is preferably combined with push key I) as illustrated by the dotted line therebetween. Each of the subordinate stations B, C, and D comprises a battery 6, a buzzer I, a hookswitch 8, a receiver 9 and associated coupling transformer II], a transmitter II, which is preferably combined with receiver 9 into a handset, an inductance coil I2, and a condenser I3.

When a party at master station A desires to communicate with a party at one of the subordinate stations the corresponding push key is momentarily operated. Assumin that push key 11 is operated, contacts bl,v transfer conductor I6 from condenser 5 to conductor I5 thus completing an obvious circuit to the buzzer "I at station 3 in series with the battery 6 thereat. The party at station B responds to the call by removing the handset from the hookswitch 8. The closure of the upper contacts of hookswitch 8 at station B completes a direct current circuit to the transmitter II thereat in series with battery '6 and inductance coil I2 and completes a direct current circuit to transmitter I at the master station over conductors I4 and I5 in series with battery Gand the left winding of transformer I0 at station B. The closure of the lower contacts of hookswitch 8 completes an alternating current path between transmitter II and the right winding of transformer 4 at the master station over conductors I5 and I6 and in series with condensers 5 and I3. The party at station B then speaks into the transmitter II in reply to the call which causes corresponding speech currents to be transmitted over the above traced alternating current path to the transformer 4, which currents are audibly reproduced by the loudspeaking receiver 3 at the master station. The party at the master station may speak into transmitter I to cause corresponding speech currents to be transmitted over conductors I4 and I5 t0 the coupling transformer II], which currents are aubidly reproduced by the receiver 9 at station B. The buzzers I at the subordinate stations are of the high impedance type so as to prevent any appreciable portion of the speech currents produced by the subordinate station transmitters I I from being shunted thereby.

If the party at one of the subordinate stations desires to communicate with a party at the master station A, the handset at the subordinate station is removed from the hookswitch thereat which completes a pair of speech channels between the calling subordinate station and the master station as described above when a call initiated at the master station was answered at subordinate station E. The party at the calling subordinate station signals the master station by speaking into his transmitter H, the speech signals being audibly reproduced by the loudspeaking receiver 3 at the master station to attract the attention of the party thereat.

A party at one of the subordinate stations may remove his handset from its hookswitch without speaking into the transmitter with the intention of eavesdropping on the master station. To prevent such eavesdropping the turn key I) may be operated to disconnect transmitter I from conductor M at contacts 122. These contacts connect buzzer 2 to conductor M in place of trans mitter I so that the buzzer 2 will operate when a handset at any one of the subordinate stations 4 ceiving and transmitting means, respectively, at said other station.

2. In a, telephone system as claimed in claim 1, a second switch at said one station operated to disable the transmitting means thereat to prevent eavesdropping by a party at said other station.

3. In a telephone system, a first station including a transmitter, a signal device, and a pair of contact spring sets, a second station, a line extending between said stations, one of said contact spring sets being operative to connect either said transmitter or said signal device to said line, the other of said contact spring sets being operative to signal said second station over said line, and a single switch which operates one of said contact spring sets when moved rectilinearly and operates the other of said contact spring sets when rotated.

is removed from its hookswitch, thereby reminding the party at the master station to restore turn key I) when a call is initiated by a-subordinate station or when a subordinate station answersa call initiated by the master station.

Although separate keys may be used to operate contacts bl and b2 a more compact and better appearing unit results when ,these contacts are arranged to be operated by a combination push and turn key as illustrated in Fig. 2. In the construction shown, two parallel spaced apart plates 7 l9 and are secured together by a pair of transthe plungers are depressed. These contact spring sets are secured to the plate 20 in cantilever fashion. Plunger b has an insulated can member b of rectangular box form secured thereto and in engagement with the armature spring of contact spring set b2, which is secured to the transverse plate 2i in cantilever fashion. When plunger 2) is rotated the cam member b operates contact spring set 172 independently of contact spring set bl.

While a particular embodiment of the invention has been described, it is to be understood that numerous modifications in the construction and arrangement of parts may be resorted to without departing from the true spirit and scope of the invention as defined in the appended claims.

What is claimed is:

1. In a telephone system, a pair of stations, three conductors extending between said stations, a transmitting means connected between two of said conductors and a condenser and a receiving means serially connected between one of said two conductors and the third conductor at one of said stations, a switch operated to complete a direct current circuit between said one and said third conductors at'said one station, a signal device and a source of direct current serially connected between said one and said third conductors at the other of said stations, said signal device being operated in response to the operation of said switch, a hookswitch, transmitting means, and a receiving means at said other station, said hookswitch being operated to complete energizing circuits to both of said transmitting means from said source and to couple the transmitting and receiving means at said one station to the resubordinate station operated.

4. In a telephone system, a master station and a plurality of subordinate stations, a pair of in- V terstation conductors common to all of said stations, an individual conductor extending between each of said subordinate stations and said master station, a signal device at each subordinate station connected between the'conductor individual thereto and'one of said common conductors, switching means at said master station operated to selectively energize any of said signal devices over the individual and common conductors connected thereto to call a subordinate 'statiomand switching means at each subordinate station operated when a call is answered thereat to complete two separate and oppositely directed speech channels between the master station and the called subordinate station over the three of said conductors extending therebetween.

5. In a telephone system as claimed in claim 4, switching means at said master station at times operated to disable one of said speech channels.

6. In a telephone system as claimed in claim 4, a transmitter at each of said stations, a source of current at each of said subordinate stations, said switching means at each subordinate station completing energizing circuits to'the transmitter 'at said master station over said common conductors and to the'transmitter at the called from the source thereat when '7. In a telephone system as claimed in claim 4, a second switching means at said master station at times operated to disable one of said speech channels, and a common actuating means at said master station moved rectilinearly to operate a part of said first switching means thereat and rotated to operate said second switching means thereat.

8. In a telephone system, a master station and a plurality of subordinate stations, three conductors extending between each of said subordi- Irate stations and said master station, a signal device at each subordinate station, switching means at said master station operated to selectively energize any of said signal devices over a pair of said conductors to calla subordinate station, and switching means at each subordinate station operated when a call is answered thereat to complete' a pair of independent and oppositely directed speech channelsbetween said m'asterstaa transmitting means connected between two of said conductors and a condenser and a receiving means serially connected between one of said two conductors and the third conductor at one of said stations; a source of direct current connected between said one and said third conductors at the other of said stations, at transmitting means and a receiving means at said other station, means to complete energizing circuits to both of said transmitting means from said source and to couple the transmitting means at said one station to the receiving means at said other station thereby completing a direct current circuit over two of said conductors and to couple the receiving means at said one station to the transmitting REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,802,675 Sisty et a1 Apr. 28, 1931 1,351,775 Long Sept. 7, 1920 

